Preliminary Report on the Phytal Animals in the Sargassum Confusum Region in Oshoro Bay, Hokkaido (With 6 Text- Title Figures and 1 Table)
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Preliminary Report on the Phytal Animals in the Sargassum confusum Region in Oshoro Bay, Hokkaido (With 6 Text- Title figures and 1 Table) Author(s) KITO, Kenji Citation 北海道大學理學部紀要, 20(1), 141-158 Issue Date 1975-10 Doc URL http://hdl.handle.net/2115/27597 Type bulletin (article) File Information 20(1)_P141-158.pdf Instructions for use Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP Preliminary Report on the Phytal Animals in the Sargassum confusum Region in Oshoro Bay, Hokkaido1) By Kenji Kito Zoological Institute, Hokkaido University (With 6 Text-figures and 1 Table) Marine seaweed regions harbour rich fauna of many different kinds of animals, and such animals life long or temporarily living in, on or among seaweeds are generally called "phytal animals". Numerous important studies on seaweed communities including these phytal animals have been so far carried out mainly in European seas from various biological view-points by many authors (Colman, 1940; Dahl, 1948; Wieser, 1952; Chapman, 1955; Ohm, 1964; Hagerman, 1966; Nagle, 1968; and other authors). On the other hand, in Japan, seaweed communities were studied mainly in the Zostera and the Sargassum regions by Kitamori and Kobayashi (1958), Kitamori et al. (1959), Kikuchi (1961, 1962 and 1966), Fuse (1962a and b), Harada (1962) and others mainly from the view-point of fishery resources, especially for the relation between fishes and phytal animals as their food organisms. In these studies, however, most meio- or microorganisms, such as harpacticoids, nematodes, etc., were omitted or disregarded despite of their predominant existence in these communities. Therefore, Mukai (1971) recently made a quantitative investigation of phytal animals including meiofauna in the Sargassum region in the Inland Sea of Japan and described their seasonal fluctuations of abundance. His study was carried out mostly at higher taxonomic level, but it seems clearly desirable that such quantitative studies in the seaweed community are carried out rather at each species level and further in more various localities of different kinds of environment, particularly to clarify the precise structure and function of the seaweed community. In the present study, with this ecological significance in mind, I treat the 1) Biology of phytal animals in the Sargassum confusum region in Oshoro, 1. Jour. Fac. Sci. Hokkaido Univ. Ser. VI, Zool. 20 (1), 1975. 141 142 K. Kito phytal animals in the Sargassum region in Oshoro Bay, Hokkaido, northern Japan. Since no study on this field has been carried out in this locality, clarification of general ecological aspect of the animals was the primary and main problem for this study. In this paper, seasonal change of faunal compositon and populations of non-sessile phytal animals is described as the first report from the present investiga tion, leaving detailed results elsewhere. Methods 1. Areas surveyed: The investigation was carried out in the Sargassum region in Oshoro Bay (Fig. I), about 15 km west of Otaru, on the west coast of Hokkaido, northern Japan. Oshoro Bay is under the influence of the warm Tsushima Current during summer to autumn, while under that of the cold Liman Current in the other seasons. The bay is mainly formed out of rocky shore except for the inner part. In Oshoro Bay various algae form their luxuriant regions in the intertidal and subtidal zones. Among these regions, the Sargassum region is the most abundant and remarkable. The Sargassum region consists of some species of Sargassum and is formed on the hard sea·bottom distributed in various places within the subtidal zone (Fig. 1). The area indicated by an arrow was selected for the present work because this area was seemed moderate and was composed of rather pure growth of Sargassum confu8um Agardh. In this area water temperature and specific gravity as environmental factors were measured at every sampling time. The natural environment and faunal and floral accounts of Oshoro Bay have been reported by Motoda (1971). 2. Sargassum surveyed: Sargassum confu8um provides various habitats for phytal animals and, therefore, its changes of the shape with their growth and the seasonal variation of standing crop in the Sargassum region immediately influence their lives. For this reason the mean wet weight per plant and the standing crop were calculated every sampling month. First, the number of S. confusum/m' bottom was estimated by the mean density of 5 quadrates (50 X 50 cm'). And, to estimate growth and standing crop of plant, all the individuals of S. confusum in one quadrate or two were collected and the length, volume, wet and dry weight, and so on were measured for all plants. In this paper the mean wet weight per plant and the standing crop are also used to quantify the individual numbers/m' bottom. 3. Sampling procedure and treatment of animals: The periodical sampling was made at daytime in the Sargas8um region, once a month, from August in 1973 to July in 1974. Samples were quantitatively taken by using a bag as like to that adopted by Mukai (1971), which is 25 cm square at the base, 100 cm long, and made of XX 13 gauze (0.0095 mm meshes). Wearing a face mask and a snorkel, the collector enveloped a plant of Sargas8um with this bag, taking care to keep phytal animals, and cut off the discoid holdfast from rock surface. As soon as this procedure was over, he closed the mouth of the bag, and brought it to the sea surface. Phytal animals except microfauna could be taken together with a plant by this method. Samples at each time were of two to five plants collected at random and were preserved in 5% neutralized formalin solution. In the solution each plant was shaken enough to release the animals from it, and macro· fauna such as molluscs, some amphipods, decapods, etc. among the animals remained in it Phytal Animals in Sargassum Region 143 t Japan Sea Oshoro Boy Oshoro Bo), , so ••• r. toru· O~Om Oshoro Marine Biological Stotion Fig. 1. Location of Oshoro Bay and distribution of the Sargas8um region (dotted area). The area surveyed is indicated by arrow. were counted under a binocular microscope, and meiofauna was estimated by counting the number of individuals appearing in each 1/100 subsample. Furthermore, larger animals caught from plant were sorted by hand and counted for the total. The number of animals per plant was totaled from the number of hand sorting and that calculated on the bases of 5 subsamples (Appendix 1). At the same time, length, volume, wet and dry weight of each plant were measured. In the present investigation, some epiphytes attached on S. confusum were regarded as parts of Sargassum and protozoans and fishes were not treated because their precise amount seemed to be unable to be estimated by this method. 4. Estimation of the number of phytal animals: First, mean population density/g algae was calculated from the individual number/plant described. And individual number 1m2 bottom was estimated from its mean density of animals/g algae and the standing crop of Sarga88Um confU8um in each month. In this paper seasonal fluctuations of phytal animals are dealt with by these two indices, individuals/m' bottom and /g algae. Results and Discussions 1. Environmental conditions: Water temperature and specific gravity were measured in the sampling area at every time. Seasonal variations of these environmental factors are shown in Fig. 2. Surface temperature ranged from 3.1 °C in February to 26.0°C in August in the sampling year. Specific gravity of surface water (0'15) ranged from 1.0202 in May to 1.0259 in July. The decrease in April and May is caused by the inflow of snow melting water in early spring and 144 K. Kito 30 ~ 0;; 20 1.025 '""'_ ~ ,., ..E! "> Co E! E '" !? u 10 '"U "0" Co ~ Vl" 1.020 Fig. 2. Seasonal changes of water temperature (solid circles) and specific gravity (open circles) in the Sarga88um region. in the other months specific gravity is nearly constant. The seasonal variations of mean length, mean wet weight, mean density and standing crop of Sargassum confusum are shown in Fig. 3 and Appendix 2. The minimum wet weight was found in September (24.3 g), and till January it nearly remained. Since February it tended to increase and reached the maximum in May (157.2 g), and then it decreased rapidly in June and July. The pattern of the seasonal variation of mean length nearly agreed with that of mean wet weight except that the minimum occurred in October (27.7 cm) and the maximum in June (117.9 cm). As shown in Fig. 3, the number of plants of S. confusumjm2 bottom was 77.2 at maximum in September, though from November to June it seemed to be stable (mean 40.5 plants). At last, standing cropjm2 bottom was calculated from mean wet weight and density of plants. Standing crop was poor during September and the following four months and was about 1.4 kg/m2 bottom at minimum in January. Since February it increased and reached the maximum also in May (about 6.4 kg/m2 bottom). As a result, seasonal change in growth of Sargassum confusum is divisible roughly into the following four seasons. Stable season (abr. S.S.), November to the middle of January: The dislodgement by wave action and withering is entirely finished and the density becomes stable (40.5 plantsjm2 bottom in the present survey). In this season a plant of Sargassum consists mainly of a primary stem and principal branches with large basal leaves, and these upper portions of the principal branches seem to elongate upward slightly. But the growth in length and wet weight is generally insignificant, therefore, the standing crop is the lowest during the year (about 1.41 to 1.80 kgjm2 bottom).